When you add field to attachment, it only shows in the attachment uploading screen:
Though it doesn’t show in “image details”, making it really hard to update the value after the image is added to the post:
To edit the field, you must go to Media -> Library -> Attachment.
I understand it’s not that easy since you’ll have to mess with JS (that screen is rendered completely with js), but otherwise the whole idea of attachment field is not that attractive anymore.
This is a real issue if planning to add a bunch of advanced customs fields to images for content creators to enhance image data. Once they have inserted the image into the post, there is no way for them to edit that data without opening up the media library, finding the image and editing it there, which is not really tenable, first because it is a atrocious workflow, and secondly it is an advanced step to teach content creators, especially on blogs with multiple editors/authors who aren’t tech/wordpress savvy.